In recent years, large flat television receivers using plasma display panels or liquid crystal panels as the display units are becoming increasingly popular. In line with this, sound reproducing devices to be mounted on such television receivers are required to be smaller and thinner. At the same time, home theaters, terrestrial digital broadcasting, and other media are becoming popular and improving in sound quality. This trend requires the sound reproducing devices to have high-quality sound and high maximum input power.
In order to meet the demand for high-quality sound, low-frequency sound reproducing technology is inevitable which reproduces a powerful sound for movies and the like. One example of the low-frequency sound reproducing technology is a loudspeaker device having a passive radiator.
A conventional loudspeaker device having a passive radiator is shown in FIG. 5 in a sectional side view. As shown in FIG. 5, loudspeaker device 100 includes loudspeaker box 101 having two openings on its front surface. Loudspeaker box 101 includes loudspeaker unit 102 attached to one opening and passive radiator 103 attached to the other opening. Loudspeaker unit 102 includes magnetic circuit 102a. Passive radiator 103, which includes a flat diaphragm, is attached to loudspeaker box 101 via edge 103a. 
Passive radiator 103 is driven by exhaust pressure to be applied to loudspeaker box 101 at the time of driving loudspeaker unit 102. This amplifies the sound reproduction in a predetermined low frequency region. One such conventional loudspeaker device 100 is disclosed in Japanese Utility Model Unexamined Publication No. S57-2790 (Patent Document 1).    Patent Document 1: Japanese Utility Model Unexamined Publication No. S57-2790